Memory systems, such as storage systems, may be implemented in electronic systems, such as computers, cell phones, hand-held electronic devices, etc. Some storage systems, such as solid state drives (SSDs), embedded Multi-Media Controller (eMMC) devices, Universal Flash Storage (UFS) devices, and the like may include non-volatile storage memories for storing host (e.g., user) data from a host. Non-volatile storage memories provide persistent data by retaining stored data when not powered and may include NAND flash memory, NOR flash memory, read only memory (ROM), Electrically Erasable Programmable ROM (EEPROM), Erasable Programmable ROM (EPROM), and resistance variable memory, such as phase change random access memory (PCRAM), three dimensional cross-point memory (e.g., 3D XPoint), resistive random access memory (RRAM), ferroelectric random access memory (FeRAM), magnetoresistive random access memory (MRAM), and programmable conductive memory, among other types of memory.
Memory cells are often arranged in an array architecture and can be programmable to a number of different data states corresponding to one or more data units (e.g., bits). As an example, some memory cells, such as some resistance variable memory cells, can be programmed to low resistance state, corresponding to a low threshold voltage (Vt) state or a high resistance state, corresponding to a high Vt state. In some examples, a resistance variable cell in a lower resistance state can be said to be in a set state, corresponding to a set Vt distribution (e.g., encoded as a logical 1), and a resistance variable cell in a higher resistance state can be said to be in a reset state, corresponding to a reset Vt distribution (e.g., encoded as a logical 0).
Resistance variable memory cells can be rewritten by overwriting them without first erasing them, in some examples. This can be referred to as writing in place. In some examples, resistance variable memory cells can be written out of place to a new location after the new location has been “cleaned” by writing all the cells previously in the set state to the reset state, so that all the cells are in the reset state.
The state of a memory cell can be sensed (e.g., read) by, for example, determining whether the cell changes its resistance state (e.g., experiences a switching event) in response to applying a sensing voltage, that may be referred to as a read voltage or a demarcation voltage, to (e.g., across) the cell. For example, the sensing voltage can be selected to lie in a range of Vts (e.g., a Vt window) between a lower edge of the reset Vt distribution and an upper edge of the set Vt distribution.